Field numbers on plain data classes, no generated layer

A schema compiler emits code for one platform. In a multiplatform module that is fatal: the generated types compile on the JVM and strand every other target, so the whole protocol layer ends up behind expect/actual or duplicated. The alternative is to write the messages by hand and carry the field numbers as annotations, letting the serialization library produce the same bytes:

// adapted — the message names, field names and numbering are placeholders; the mechanism is real
@Serializable
data class Frame(
    @ProtoNumber(1) val kind: String = "",
    @ProtoNumber(2) val body: ByteArray = ByteArray(0),
    @ProtoNumber(3) val packed: Boolean = false,
)

@OptIn(ExperimentalSerializationApi::class)
private val proto = ProtoBuf { encodeDefaults = false }